Electrical network automatically responsive to a change in condition



June 1962 e. A CUTSOGEORGE ETAL 38,

ELECTRICAL NETWORK AUTOMATICALLY RESPONSIVE TO A CHANGE IN CONDITIONFiled Dec. 12, 1958 INVENTOR 5 George ,4. Catsogacrge Lester V.l-lebenstrei 1 BY uh'lliamJ 5 av; Mild;

AGENT United States Patent 9 l 3,038,106 ELECTRICAL NETWORKAUTOMATICALLY RE- SPONSIVE TO A CHANGE IN CONDITION George A.Cutsogeorge, Cranford Township, Union County, Lester V. Hehenstreit,Bloomfield, and William J. Spaven, North Arlington, N.J., assignors toSpecialties Development Corporation, Belleville, N.J., a corporation ofNew Jersey Filed Dec. 12, 1958, Ser. No. 780,061 6 Claims. (Cl.317148.5)

The present invention relates to electrical networks which areautomatically responsive to a change in a condition, and, moreparticularly, to such networks which are responsive to the rate ofchange of such condition and are prevented from giving false indicationsin response to transient conditions.

The present invention, although useful for many other purposes, isprimarily concerned with improving heat and flame detecting systems ofthe type shown in co-pending application for Letters Patent of theUnited States, Serial No. 624,074, now Patent No. 2,901,740, filedNovember 23, 1956, and assigned to the assignee of this application.Such systems include a thermistor element which comprises two conductorsspaced apart by a material having an infinitely high resistance at anormal temperature to render it substantially non-conductive toelectricity and having the characteristic of being rendered conductiveat an abnormal temperature. The thermistor element forms one leg of aresistance bridge which is connected to circuitry for giving an alarmwhen the resistance of the element is such as to indicate the presenceof an abnormal temperature condition.

The particular system disclosed in the above mentioned co-pendingapplication also includes a rate circuit and therefore gives anindication not only when a given temperature is detected but also when agiven rate of change of temperature is detected. This system is used inaircraft, and, while it is satisfactory in many respects, it has beenfound to be subject to false alarms due to various transient conditionswhich produce a sufliciently rapid change in the balance of the bridgeto cause the rate circuit to actuate the alarm. Such transientconditions include the condensation of moisture on the element, and thedischarge of static electricity through the element. These conditionshave no relation to a fire hazard and therefore it is highly desirablethat the resulting false alarms be eliminated.

Accordingly, an object of the present invention is to provide a networkof this type which is not subject to the foregoing difficulties.

Another object is to provide such a network which is prevented fromresponding to a rate of change in the detected condition if the changeis instantaneous.

Other and further objects of the invention will be obvious upon anunderstanding of the illustrative embodiment about to be described, orwill be indicated in the appended claims, and various advantages notreferred to herein will occur to one skilled in the art upon employmentof the invention in practice.

In accordance with the present invention the foregoing objects areaccomplished by providing in a condition responsive system whichincludes a source of unidirectional current, first and second resistanceelements connected in series across the source, third and fourthresistance elements connected in series across the source, the elementshaving resistance values to provide a voltage dividing bridge normallyunbalanced in one direction and one of the resistance elements beingconstructed and arranged so that the resistance thereof changes inresponsive to a predetermined condition to another value whereby thebridge goes towards unbalance in the opposite direction, an elec-3,038,106 Patented June 5, 1962 ICC tronic valve having an inputelectrode connected to the junction of the first and second resistanceelements and having another input electrode connected to the junction ofthe third and fourth resistance elements and having an output electrodeconnected for supply of current thereto, a capacitor providing at leastin part the connection between the junction of the first and secondresistance elements and the first mentioned input electrode fordetecting the rate of change of the condition of the bridge with respectto balance, an electrically operable device connected to be controlledby the output of the electronic valve, means connected to the device fordelaying the operation of the device after the output of the electronicvalve is such as to cause operation of the device, and means providing alow resistance discharge path for the capacitor so that the device willnot operate in response to the rate of a change in the condition of thebridge when the change is instantaneous.

A preferred embodiment of the invention has been chosen for purposes ofillustration and description, and is shown in the accompanying drawing,forming a part of the specification, wherein:

The single FIGURE of the drawing is a circuit diagram of a conditionresponsive network in accordance with the present invention.

Referring to the drawing in detail, there is shown a network inaccordance with the invention which generally comprises a source ofunidirectional electrical current having its negative side connected toa conductor 10' and its positive side connected to a conductor 11, athermistor element 12 (first resistance element) and a 10,000 ohm fixedresistor 14 (second resistance element) connected in series between theconductors 10 and 11, and a 3900 ohm fixed resistor 15 (fourthresistance element) and a 500 ohm resistor 16 (third resistance element)connected in series across conductors 10 and 11 to form a resistancebridge with the element 12 and the resistor 14. The resistor 16 isprovided with an adjustable tap 17, and the output of the bridge, whichindicates the condition of the bridge with respect to balance, is sensedbetween this adjustable tap 17 and a point X at the junction of theelement 12 and the resistance 14.

An NPN type junction transistor 19 having a base 20, an emitter 21, anda collector 22, has its input circuit connected across the bridge tosense the bridge output, the emitter 21 being connected to the tap 17through a 10 ohm current limiting resistor 24 and the base 20 beingconnected to the point X through a rate sensitive and temperaturecompensating network 25. The network 25 comprises a diode 26, a 50microfarad capacitor 27, and a 100,000 ohm resistor 28 all connected inparallel. The base 20 is also connected to the positive conductor 11through a 120,000 ohm resistor 30, and the collector 22 is connected tothe positive conductor 11 through a 10,000 ohrn load resistor 31.

The output of the transistor 19 controls a second NPN type junctiontransistor 32 having a base 34 connected to the collector 22 oftransistor 19, an emitter 35 connected to the emitter 21 of thetransistor 19 through a 470 ohm feedback resistor 36, and a collector 37connected to the positive conductor 11 through a relay winding 39 whichcontrols a switch 40 in an external indicating circuit. The emitter 35is also connected to the junction of a 910 ohm resistor 41 and a 270 ohmresistor 42 arranged in series between the conductors 10 and 11.

In the operation of the portion of the network just described, when thethermistor 12 is at normal temperature its resistance is very high(60,000 to 500,000 ohms) therefore substantially all of the sourcevoltage is dropped across the thermistor 12, and the point X has a veryhigh positive potential. The tap 17, on the other hand, has a lowpositive potential since the resistor 16 on which it is located has alow resistance in comparison to that of the resistor 15. The base 20 ofthe transistor 19, therefore, is more positive than the emitter 21 andcurrent flows from the conductor 11 both through the resistors 14 and 28and through the resistors 30 to the base 20, and through the baseemitter circuit of the transistor and the resistors 24 and 16 to theconductor 10. This flow places the transistor 19 in full conduction andallows current to flow from the conductor 11 through the load resi tance31, the collector emitter circuit of the transistor, and the resistors24 and 16 to the conductor 10. With the transistor 19 in full conductionthe collector 22 is less positive (with respect to the conductor thanthe junction point of resistors 41 and 42 since a greater portion of thesource voltage is dropped across resistor 31 than is dropped acrossresistor 41. The base 34 of transistor 32 is therefore less positivethan the emitter 35 and the transistor 32 is held in a cutoif condition.

As the resistance of the thermistor 12 decreases in response to anincrease in temeprature, the potential of the point X decreases and thecurrent supplied to the base 20 from the point X (through resistor 14)likewise decreases. The current flowing to the base 20 through theresistor 30, however, remains substantially constant until the point Xbecomes less positive than the base 20 allowing the diode 26 to conduct.

When the point X does become less positive than the base 20, some of thecurrent flowing through the resistor 30 is shunted away through thediode 26 reducing the base current of the transistor 19. At somepredetermined temperature condition the balance of the bridge will besuch that suificient current will be diverted from the base circuit toreduce the collector current to a point where the base 34 of transistor32 will be more positive than the emitter 35 and the transistor 32 willbegin to conduct.

The exact condition of the bridge with respect to balance which willreduce the collector current in the transistor 19 to the operating levelis dependent upon the position of the tap 17 on the resistor 16 and uponthe relation of the resistance of the diode 26 to the base-emitterresistance of the transistor 19. Since both the diode and the transistorare semi-conductors, their resistances vary with temperature in the samemanner, therefore, the point of operation does not vary appreciably withtemperature.

When the transistor 32 beings to conduct, the potential of the emitter35 (with respect to the conductor 10) increases due to the increasedcurrent flow through the resistor 42. This increase in potential is fedback to the emitter 21 of the transistor 19 through the resistor 36making the emitter 21 more positive than the base 20, cutting off thetransistor 19. The transistor 32 then goes into full conductionenergizing the relay coil 39 to close the switch 40 and give anindication in the external circuit that a fire or dangerous heatcondition is present.

In order that a flash or rapid fire may be detected before it reachesdangerous proportions diflicult to extinguish, the rate portion of thenetwork 25 opreates when the element is exposed to a rapid increase intemperature, to give an indication before the thermistor is heated to atemperature suflicient to balance the bridge.

The potential at the base 20 controls the conduction of the transistor19 and is at all times equal to the source voltage minus the IR dropacross the resistor 30. The rate portion of the network 25 aifects theconduction of the transistor 19 by influencing the voltage drop acrossthe resistor 30 as follows.

When the resistance of the thermistor element is high and constant, thediode 26 is not conducting, the transistor 19 is conducting, thecapacitor 27 is charged to the voltage drop across the resistor 28, andthe voltage drop across the resistor 30 is constant and proportional tothe current flowing therethrough to the base 20.

As the element 12 decreases in resistancee slowly in response to anormal increase in ambient temperature, the capacitor 27 dischargesthrough the resistor 28 at a 4- rate sufficient to maintain its chargeequal to the IR drop across the resistor 28. Under these conditions thevoltage drop across the resistor 39 remains constant holding thepotential at the base 20 constant.

However, when the resistance of the element 12 drops rapidly, thecapacitor 27 cannot discharge through the resistor 28 at a comparablerate and therefore an appreciable discharge current also flows from thepositively charged plate of the capacitor through the element 12 and thepower supply, and through the resistor 14 in parallel with the elementand the power supply, to the conductor 11 and through the resistor 30 tothe negatively charged plate. This dischargee current increases the IRdrop across the resistor 30 thereby decreasing the positive potential onthe base 20. It may be seen that if the discharge of the capacitor 27increases the current through the resistor 30 to a point where the IRdrop across the resistor is greater than the source voltage, the base2!] will be driven negative with respect to ground.

This circuit is arranged so that when the resistance of the element 12decreases rapidly to a value of 30,000

ohms or less, the rate capacitor 27 causes the transistor 19 to be cutoff.

The network described so far in detail is disclosed in theaforementioned application. It has been found that the transientconditions mentioned hereinabove cause instantaneous changes in thebalance of the bridge of sufficient magnitude to cause the rate circuitto drive the transistor 19 to cut off. In the network of theaforementioned application, the transistor 32 is placed in conduction bythis action and continues to conduct until the capacitor 27 dischargessufficiently to allow the transistor 19 to conduct once again. In thenetwork described hereinabove in detail, an instantaneous change in thebalance of the bridge causes the capacitor 27 to hold the transistor 19cutoff for a period of about three seconds.

In accordance with the present invention, this network is prevented fromgiving an alarm in response to such transient conditions by the additionof a microfarad capacitor 44 connected in series with a 1,000 ohmresistor 45 between the collector 37 and the base 34 of transistor 32,and a diode 46 connected between the emitter 21 and the base 24 oftransistor 19 in a manner such that its conductive direction is oppositeto that of the baseemitter circuit of the transistor. Then, in standbyoperation when the transistor 19 is conducting, holding the transistor32 cutoff, the diode 46 is biased against conduction, and the capacitor44 is charged to a potential equal to the diiferencee between thepotential of conductor 10, present at the collector 37, and thepotential at the collector 22. When the transistor 19 is driven tocutoff by a rapid change in the balance of the bridge, the collector 22is temporarily prevented from rising to the line potential by the chargeon the capacitor 44. The transistor 32, therefore, is prevented fromconducting for a period of time dependent on the rate of discharge ofthe capacitor 44. In the illustrative embodiment described hereinabovethis time period is one second.

During the above operation, when the rate network drives the transistor19 to cutoff, the voltage biasing the diode 46 against conduction isremoved and the capacitor 27 discharges rapidly through the lowresistance path provided by the element 12, the conductor 10, theresistors 16 and 24, and the diode 46. The rate network is therebyprevented from holding the transistor 19 cutoff for a period exceedingthe delay provided by the capacitor 44 when the change in the balance ofthe bridge is instantaneous. 1n the preferred emebodiment describedhereinabovee the resistance of this discharge path is such that anyinstantaneous change in the balance of the bridge will not cause thecapacitor 27 to cut off the transistor 19 for a period exceeding 0.75second.

Therefore, since the transistor 32 will conduct only if the transistor19 remains cut 011 for a period of more than one second, the networkresponds to a rate of change in the balance of the bridge only if thisrate of change continues for a predetermined time.

It will be seen that the operation of the rate network is notappreciably affected by the addition of the diode 46 since the diode isbiased against conduction when the transistor 19 is conducting andtherefore has no function until the bias is removed when the transistor19 is cut off.

The resistor 45 is provided to limit current flow through the relay coil39 when the capacitor 44 is charging, thereby preventing the coil 39from being energized momentarily, giving a false indication, whenthenetwork is initially energized or is reset after giving a fireindication.

From the foregoing description, it will be seen that the presentinvention provides a condition responsive network which is preventedfrom responding to a rate of change in the detected condition if thechange is instantaneous.

As various changes may be made in the form, construction and arrangementof the parts herein, without departing from the spirit and scope of theinvention and without sacrificing any of its advantages, it is to beunderstood that all matter herein is to be interpreted as illustrativeand not in any limiting sense.

We claim:

1. In a condition responsive system including a source of unidirectionalcurrent, first and second resistance elements connected in series acrosssaid source, third and fourth resistance elements connected in seriesacross said source, said resistance elements having resistance values toprovide a voltage dividing bridge normally unbalanced in one directionand one of said resistance elements being constructed and arranged sothat the resistance thereof changes in response to a predeterminedcondition to another value whereby the bridge goes towards unbalance inthe opposite direction, an electronic valve having an input electrodeconnected to the junction of said first and second resistance elementsand having another input electrode connected to the junction of saidthird and fourth resistance elements and having an output electrodeadapted to be connected for supply of current thereto, a capacitorproviding at least in part the connection between the junction of saidfirst and second resistance elements and the first mentioned inputelectrode for detecting the rate of change of the condition of thebridge with respect to balance, an electrically operable device adaptedto be connected for supply of current thereto and connected to becontrolled by the output of said electronic valve: the improvement whichcomprises means connected to said device for delaying the operation ofsaid device after the output of said electronic valve is such as tocause operation of said device, and means providing a low resistancedischarge path for said capacitor so that said device will not operatein response to the rate of a change in the condition of the bridge whenthe change is instantaneous.

2. A system according to claim 1, wherein said last mentioned means is adiode connected between the input electrodes of said valve in a mannersuch that its conductive direction is opposite to the conductivedirection of the input circuit of said valve, whereby a low resistancedischarge path is provided for said capacitor.

3. A system according to claim 1, wherein said electrically operabledevice includes a second electronic valve having a pair of inputelectrodes one being connected to the output electrode of said firstvalve and having an output electrode adapted to be connected for supplyof current thereto, and said delaying means includes a capacitorconnected between said output electrode of said first valve and saidoutput electrode of said second valve.

4. A system according to claim 3, wherein a relay coil is connected tosaid output electrode of said second valve, and a resistor is connectedin series with said delaying means capacitor to limit th chargingcurrent flowing to said capacitor through said relay coil.

5. In a condition responsive system including a source of unidirectionalcurrent, first and second resistance elements connected in series acrosssaid source, third and fourth resistance elements connected in seriesacross said source, said resistance elements having resistance values toprovide a voltage dividing bridge normally unbalanced in one directionand one of said resistance elements being constructed and arranged sothat the resistance thereof changes in respons to a predeterminedcondition to another value whereby the bridge goes towards unbalance inthe opposite direction, on electronic valve having an input electrodeconnected to the junction of said first and second resistance elementsand having another input electrode connected to the junction of saidthird and fourth resistanc elements and having an output electrodeadapted to be connected for supply of current thereto, a capacitor and aresistor connected in parallel between the junction of said first andsecond resistance elements and the first mentioned input electrode fordetecting the rate of change of the condition of the bridge with respectto balance, a second electronic valve having a pair of input electrodesone being connected to the output electrode of said first electronicvalve and having an output electrode adapted to be connected for supplyor" current thereto: the improvement which comprises a capacitorconnected between said output electrode of said first valve and saidoutput electrode of said second valve for delaying the operation of saidsecond valve after the output of said first electronic valve is such asto cause said second valve to conduct, and a diode connected betweensaid input electrodes of said first valve in a manner such that itsconductive direction is opposite to the conductive direction of theinput circuit of said first valve thereby providing a low resistancedischarge path for said first capacitor so that said second valve willnot conduct in response to the rate of a change in the condition of thebridge when the change is instantaneous.

6. A system according to claim 5, wherein a relay coil is connected tosaid output electrode of said second valve, and a resistor is connectedin series with said delaying means capacitor to limit the chargingcurrent flowing to said capacitor through said relay coil.

References Cited in the file of this patent UNITED STATES PATENTS2,556,065 Callender June 5, 1951 2,777,640 Kaufman Jan. 15, 19572,864,978 Frank Dec. 16, 1953

